Context : Most of the optically classified low ionisation narrow emission-line regions ( LINERs ) nuclei host an active galactic nuclei ( AGN ) .
However , how they fit into the unified model ( UM ) of AGN is still an open question .

Aims : The aims of this work are to study at mid-infrared ( mid-IR ) ( 1 ) the Compton-thick nature of LINERs ( i.e hydrogen column densities of { N _ { H } > 1.5 \times 10 ^ { 24 } cm ^ { -2 } } ) ; and ( 2 ) the disappearance of the dusty torus in LINERs predicted from theoretical arguments .

Methods : We have compiled all the available low spectral resolution mid-IR spectra of LINERs from the InfraRed Spectrograph ( IRS ) onboard Spitzer .
The sample contains 40 LINERs .
We have complemented the LINER sample with Spitzer /IRS spectra of PG QSOs , Type-1 Seyferts ( S1s ) , Type-2 Seyferts ( S2s ) , and Starburst ( SBs ) nuclei .
We have studied the AGN versus the starburst content in our sample using different indicators : the equivalent width ( EW ) of the polycyclic aromatic hydrocarbon ( PAH ) at 6.2 \mu m , the strength of the silicate feature at 9.7 \mu m , and the steepness of the mid-IR spectra .
We have classified the spectra as SB-dominated and AGN-dominated , according to these diagnostics .
We have compared the average mid-IR spectra of the various classes .
Moreover , we have studied the correlation between the 12 \mu m luminosity , { \nu L _ { \nu } ( 12 \mu m ) } , and the 2-10 keV energy band X-ray luminosity , { L _ { X } ( 2 - 10 ~ { } keV ) } .

Results : In 25 out of the 40 LINERs ( i.e. , 62.5 % ) the mid-IR spectra are not SB-dominated , similar to the comparison S2 sample ( 67.7 % ) .
The average spectra of both SB-dominated LINERs and S2s are very similar to the average spectrum of the SB class .
The average spectrum of AGN-dominated LINERs is different from the average spectra of the other optical classes , showing a rather flat spectrum at 6 - 28 \mu m .
We have found that the average spectrum of AGN-dominated LINERs with X-ray luminosities { L _ { X } ( 2 - 10 ~ { } keV ) > 10 ^ { 41 } erg / s } is similar to the average mid-IR spectrum of AGN-dominated S2s .
However , faint LINERs ( i.e . { L _ { X } ( 2 - 10 ~ { } keV ) < 10 ^ { 41 } erg / s } ) show flat spectra different from any of the other optical classes .
The correlation between { \nu L _ { \nu } ( 12 \mu m ) } and { L _ { X } ( 2 - 10 ~ { } keV ) } for AGN nicely extends toward low luminosities only if SB-dominated LINERs are excluded and the 2-10 keV band X-ray luminosity is corrected in Compton-thick LINER candidates .

Conclusions : We have found that LINERs proposed as Compton-thick candidates at X-ray wavelengths may be confirmed according to the X-ray to mid-IR luminosity relation .
We show evidence in favour of the dusty-torus disappearance when their bolometric luminosity is below { L _ { bol } \simeq 10 ^ { 42 } erg / s } .
We suggest that the dominant emission at mid-IR of faint LINERs might be a combination of an elliptical galaxy host ( characterised by the lack of gas ) , a starburst , a jet , and/or ADAF emission .
Alternatively , the mid-IR emission of some of these faint LINERs could be a combination of elliptical galaxy plus carbon-rich planetary nebulae .
In order to reconcile the Compton-thick nature of a large fraction of LINERs with the lack of dusty-torus signatures , we suggest that the material producing the Compton-thick X-ray obscuration is free of dust .